Recent developments in mobile and wireless communication have focused on increasing the region of network coverage of a cellular network through the effective use of bridging and relaying stations. Despite such developments, it has become evident that many locations, both in domestic and commercial situations, suffer from poor wireless network provision. This can lead to disrupted or unsuccessful communication, which can be frustrating to a user. It can also lead to reduced access to high data throughput communications protocols, which limits users' access to data communications facilities such as the Internet.
In order to address this issue, the concept of femtocells has been introduced. For clarity, it should be noted that the term “femtocell” refers to establishment of wireless communication within a relatively small region of coverage (particularly when compared with traditional cellular coverage established for mobile telephony, also known as “macrocell” coverage. Exact definitions are not forthcoming and depend on environmental and regulatory factors, but it is clear that the intention is that, whereas a macrocell might be expected to span hundreds of meters of effective, useable coverage, a femtocell would only cover a range of a few tens of meters.
Communication in a femtocell is established by means of a femtocell access point (FAP). A FAP combines fixed-line broadband access with conventional wireless communication via a cellular network. It is characterised by its low transmit power and low cost femtocell access point (FAP). A FAP will be connected for broadband access with the Internet by means of a connection to the PSTN, to a cable service or to any other fixed line access service.
Although femtocell technology is nascent, it is expected that FAPs will be easy to deploy in a home or SOHO (Small Office/Home Office) environment, for instance on a “plug and play” basis. That is, a user will not be assumed by a manufacturer of a suitable device to have significant technical knowledge. This has parallels with the use of WIFI access points nowadays—installation of a WIFI access point is generally considered to be straightforward to a user able to follow a series of simple instructions and a set up “wizard”. This is not to say that a user totally unfamiliar with basic computing would find such a task straightforward—some familiarity is clearly helpful. The reader will appreciate that it is commonplace for a particular family member to be nominated as the most competent installer of computing equipment, on the basis of past experience, youth, dexterity and so on. This is not to say that another family member with less aptitude could not carry out such tasks, but that lack of familiarity is a recognised factor in contributing to unsuccessful installation.
By virtue of the fact that installation of FAPs is devolved to the home or small office user, there is no central control over location of such devices, in a macrocell. A network operator, operating base station equipment for establishing a macrocell, will have little control over the deployment of FAPs, and may need to accommodate any number (possibly in the order of hundreds) of FAPs within a location. These FAPs will inevitably not all be in optimal locations; there may be overlaps in coverage, some may be located where their own reception of the macrocell signal is somewhat compromised, and so on.
The use of a FAP to define a femtocell in a particular location is intended to provide network coverage in zones which otherwise suffer from poor or no coverage from a “macrocell” communications arrangement (for example 3G or other implementations of wireless telephony). Through provision of a FAP, a user of a suitably enabled mobile station (MS) can experience seamless communication with another network participant while that mobile user moves in or out of the resultant femtocell.
As the limit of useful coverage from a macrocell is reached (due to decreased SINR, increased likelihood of signal obstruction, and so on), a deployed FAP will provide a femtocell to which the MS's communication link can be handed over. By suitable deployment of FAPs, acceptable indoor coverage can be provided, by enabling connection of an MS to a wider network via a gateway over an existing broadband connection. A white paper by Motorola, namely “Femtocells—The Gateway to the Home”, explains this in more depth.
When an MS moves into a region having coverage both from a FAP and from a macrocell cellular connection, macrocell-to-femtocell handover could be initiated by the macrocell base station (MCBS) or the FAP. In general, an MCBS maintains a neighbour list by collecting information from MSs or neighbouring BSs. The MCBS then periodically broadcasts a neighbour advertisement which informs MSs in range in the relevant macrocell as to the identity of the or each candidate MCBS in case handover to another MCBS is required. The neighbour advertisement message could include a list of neighbouring BSs and a list of FAPs. This list of FAPs may be long, such as in the order of hundreds. It would be overly time consuming or impractical for an MS to scan through the whole list of all FAPs and BSs known to the BS, particularly as the list may include some FAPs to which the MC has no access.
An efficient handover mechanism is desirable in order to ensure seamless and pervasive communication to be realised when an MS moves from one place to another. In the femtocell environment, there are three types of handover.
Firstly, femtocell-to-macrocell handover is achieved by developing a neighbour list to be held by and managed at a FAP. This neighbour list should include not only the radio characteristics of neighbouring macrocells but also their full identities. A neighbour list may include scrambling codes and channel frequencies assigned to neighbouring macrocells and femtocells. Scrambling codes are used in CDMA to separate transmissions from different access points sharing the same channel frequency.
Secondly, femtocell-to-femtocell handover can arise when an MS moves in an indoor environment with multiple FAPs. This could occur in a multi-roomed building, in which a FAP has been installed in several rooms, for example. Femtocell-to-femtocell handover can be achieved by a FAP intercepting broadcast channel information to detect the identity of a neighbouring FAP or by using some other form of centralized configuration and information distribution.
Thirdly, macrocell-to-femtocell handover must take account of the difficulty of determining unambiguously, at a macrocell BS, the identity of a target femtocell. This difficulty arises because a large number of femtocells can be overlaid with a macrocell, even in the order of magnitude of hundreds thereof. As a result, radio characteristics reported by a mobile station (MS) may be insufficient to enable choice of a handoff target. To facilitate handoff, an MS has to determine a nearby macrocell BS or FAP from the neighbour list provided by the macrocell BS currently serving the connection with the MS.
In a conventional handover process, an MS first selects a scrambling code of a nearby access point from the neighbour list received from its current serving access point. The MS then make use of the scrambling code to decode a pilot signal which is repeatedly transmitted by the nearby access point, in order to determine the communication channel quality such as signal-to-noise ratio, or carrier to interference and noise ratio (CINR).
If the MS is satisfied with the channel quality, then it establishes a connection with the access point. Otherwise, the MS selects a different scrambling code from the neighbour list for a different access point and repeats the same process until a suitable access point has been found. Therefore, the scrambling code should be uniquely associated with one access point so that it can be uniquely identified.
However, based on the existing standards, the neighbour list length is limited. For example, in UMTS, only a maximum of 32 scrambling codes can be accommodated. Therefore, if a macrocell BS holds a neighbour list of all FAPs within its scope of communication, it is likely (given that hundreds of such FAPs may have been deployed in such a macrocell) that the scrambling code of a FAP will not be unique.
Accordingly, handover to the correct FAP might not be achieved, as any particular scrambling code may be shared by two or more FAPs. An attempt to handover to a nearby FAP which has not been authorised for use can also arise, resulting in “handover interference”. Handover interference wastes radio resources as well as the battery lifetime of the MS and hence it is undesirable.
US Patent Application US2009061892A1 suggests that handover to a femtocell can be facilitated by location information held by an MS, determined by using triangulation methods with regard to a macrocell base station or FAP. The MS can then compare the current location to stored location information of one or more femtocells to determine if one or more of the femtocells is in proximity to the MS. This is followed by scanning for the identified FAP or FAPs and connection thereto.
In that document, it is assumed that the location of the FAP is known. However, in practice, it is difficult to pinpoint the location of the FAP using existing GPS methods (such as exemplified by European Patent Application EP2051547A1). Such methods have a tendency not to be successful, particularly as GPS based location determinations can suffer due to poor signal quality in indoor environments.
International Patent Application WO2009058108A1 proposes the use of a “home profile” setting for an MS. This setting is created when a user installs FAPs. When the MS communicates with a FAP identified in the home profile, the MS adapts a neighbour listing provided by the home FAP. This simplifies the handover process as the MS has only a limited number of neighbours on the neighbour list to consider.